Model stretches when posing in blender
That means that a few vertices aren't weighted to any of your bones. Select your rig, go to Pose Mode, then select your mesh and go to Weight Paint Mode. Now select the bone that should be controlling those vertices and using the Add Brush add some weight to those vertices and they will automatically update and follow the bone you have selected. You can make two or more bones control a vertex by giving that vertex a weight from multiple bones.